Zero, or low, threshold voltage (ZVt) devices are useful in various types of integrated circuits (ICs). For example, ZVt field-effect transistors (FETs) are desirable in certain applications because of their high switching speed and low saturation voltage. ZVt FETs are useful in analog circuits, e.g., amplifiers and power supplies, and in digital circuits, e.g., for power supply decoupling in logic circuits, among other uses.
In the manufacture of semiconductor ICs, processing often begins with a p-doped wafer. Due to this p-doping, it is a relatively simple matter to form ZVt nFETs without the need to provide any masks in addition to the masks used to form the implanted wells of standard threshold voltage (Std-Vt) FETs. Since there are no additional costs needed for additional masks with respect to ZVt nFETs, Zvt nFETs may be called “free” devices. However, using conventional processing techniques, ZVt pFETs are not free devices, since they would have to be made using an additional counterdoping mask that would not be needed to form the n-well of a Std-Vt pFET. This is illustrated in FIGS. 1 and 2. FIG. 1 illustrates the step of forming a conventional n-well 20 in a p-type substrate 24 using a typical n-well mask 28. After n-well 20 is formed, a Std-Vt pFET 32 may be made by forming shallow trench isolators (STIs) 36, gate insulator 40, gate 44, source 48, and drain 52, among other things, using conventional techniques.
As shown in FIG. 2, if a ZVt pFET 56 were desired under conventional thinking, n-well 20 of FIG. 1 would have to be counterdoped to form a counterdoped region 60 beneath gate 44′ that would become the channel of the ZVt PFET. After the counterdoping has been performed, the other structures of ZVt pFET 56, e.g., shallow trench isolators (STIs) 36′, gate insulator 40′, gate 44′, source 48′, and drain 52′, among other things, may be formed using conventional techniques. This counterdoping of n-well 20 requires an additional mask 64 to mask regions of substrate 24 where counterdoping is unwanted, e.g., at all regions other than the regions of the ZVt pFETs. This additional mask 64 and associated wafer processing adds to the cost of an IC. Due to these additional costs, ZVt pFETs are not free devices. Since ZVt pFETs are not free devices, IC designers generally avoid using them. It would be beneficial if there were a method of making ZVt pFETs without additional mask and associated processing costs.